In LTE release 13, 3GPP has decided to consider the demodulation reference signal (DMRS) enhancement to better support the DL MU-MIMO. In this context, several candidate schemes are proposed to reduce mutual interference among DMRS ports.
Scheme 1: Using 12 DMRS resource elements and an orthogonal cover code (OCC) with the length of four. This scheme allows up to total 4 layers per scrambling sequence.
Scheme 2: Using 24 DMRS resource elements and using OCC with the length of 2. This scheme allows up to total 4 layers per scrambling sequence.
Scheme 2: Using 24 DMRS resource elements and using OCC with the length of four. This scheme allows up to total 8 layers per scrambling sequence.
In schemes 1 and 2, the number of orthogonal antenna ports for MU-MIMO is extended to 4 whereas in scheme 3 the number of the antenna ports is extended to 8.
For the detail implementation and configuration of schemes 1, 2 and 3, those interested readers can take a reference to TR 36.897, Study on Elevation Beamforming/Full-Dimension (FD) MIMO for LTE (Release 13) and TS 36.212, Multiplexing and channel coding (Release 12). The details would be omitted here.
Thus, in Release 13, the orthogonal DMRS port number shared by paired MU-MIMO user equipment (UEs) per scrambling sequence would be extended to 4 or even 8.
Additionally, in LTE specification, eNB sends the DMRS configuration information to UE by using the dynamic downlink control signaling (DCI). However, the current DCI for DMRS configuration only have 3 bits for indicating DMRS configuration information index, and only can support the situation where the orthogonal DMRS port number per scrambling sequence is 2.
Thus, for those alternative schemes above, the DCI in DCI format 2C/2D in the current Release 12 cannot support the extended DMRS configurations. Therefore, it is necessary to define a new DCI to support the above DMRS enhancement.